The invention relates to a process for preparing cyanuric acid by heating a solution of urea and/or biuret in a solvent while vapor is discharged.
U.S. Pat. No. 3,563,987 discloses heating urea and/or biuret to form cyanuric acid under a subatmospheric pressure of at most 250 mm Hg and at a temperature of at least 180.degree.C.
In the preparation of cyanuric acid by heating urea or biuret, the amount of the by-products ammelide and ammeline contained in the resulting cyanuric acid (hereinafter referred to as "ammelide content" for short) is of great importance. In general, an ammelide content of over 1 per cent by weight is commercially undesirable. It is common practice to purify crude cyanuric acid with an ammelide content of over 1% by weight by treating the crude cyanuric acid with a strongly acidic aqueous solution during which treatment ammelide and ammeline are hydrolyzed to cyanuric acid. However, such a hydrolysis step is very expensive when production of cyanuric acid is on a large scale basis so that it is important to avoid this step.
In these known processes it is possible to prepare cyanuric acid with an ammelide content of no more than 1 % by weight, but the art recognized processes impose several limitations on the choice of reaction conditions, so that the production yields of cyanuric acid are difficult to optimize.
It has been proposed in British Patent Specification 975,714 to prepare cyanuric acid by heating a dispersion of urea and an acidic catalyst in an inert diluent in which urea, cyanuric acid and the acidic catalyst are substantially insoluble. The ammelide content of the cyanuric-acid product obtained in this known process before applying a hydrolysis step is very high, about 3 % by weight, so that the hydrolysis step is inevitable for most purposes of cyanuric acid. A further draw-back of this known process is the fact that the cyanuric-acid product is contaminated with acidic catalyst which must be removed. A yet further draw-back is the substantial necessity to thoroughly mix the urea and the acidic catalyst prior to dispersion in the inert diluent. This known process can hardly be carried out as a continuous process.